1) Field of the Invention
The present invention relates to a technology on suppressing diffraction effect of an aperture diaphragm and a method of manufacturing the aperture diaphragm.
2) Description of the Related Art
A lens (lens-barrel) of an imaging device such as a video camera is equipped with an aperture diaphragm that has two diaphragm blades each including a recessed notch. A light quantity is adjusted by varying overlapping state between the notches. In a recent video camera, since an imaging device has a high sensitivity, when a picture or image of a subject with a high-intensity is taken, a diameter of a diaphragm opening is reduced. However, if the diaphragm opening is excessively reduced, there is a problem that a diffraction (flare) occurs and resolving power is degraded. In order to reduce an effect of the diffraction, an aperture diaphragm with a neutral density filter (hereinafter, “ND filter”) is used. The ND filter is a filter for reducing light transmittance, which is mounted on each diaphragm blade such that the ND filter covers a bottom of the notch (see, for example, Japanese Patent Application Laid Open Publication No. H8-43878).
FIG. 8 is a schematic diagram of a conventional aperture diaphragm having two diaphragm blades. ND filters 53a and 53b are mounted on diaphragm blades 51a and 51b, which constitute the aperture diaphragm, respectively. The ND filters 53a and 53b have various shapes, and an edge that forms the diaphragm opening has a convex arc shape or recessed notch. In order to form the ND filters 53a and 53b into various shapes, a sheet type material having a thickness of about 0.1 millimeters is cut out by means of die cutting using a die.
It is known that a part that is disposed on a transmission section of an incident light such as the diaphragm blades 51a and 51b, if the part has a surface in parallel to an optical axis A, causes a flare by light reflection. Even if a width of the parallel surface is about 0.1 millimeter, image quality is degraded.
FIG. 9 is a part of cross section of a lens-barrel illustrating a lens holding structure. A light shielding line 62a is formed at edges of a spacer 62, first lens 61a, and second lens 61b provided in a lens frame 60. Like an example shown in FIG. 9, the light shielding line, i.e., a continuous V-groove having a 0.5 millimeter pitch is formed at the inner diameter of parts that constitute the lens, such as the lens-barrel, the lens frame, and the spacer, which are close to the transmission section of incident light and in parallel to a transmission section, so that a portion which is in parallel to the optical axis is reduced as small as possible to prevent the occurrence of flare.
There is a tendency that thin diaphragm blades are preferably used in order to prevent the occurrence of the flare, and currently diaphragm blades made of carbon steel plates having a thickness of about 0.02 millimeter are used. A technique to remove a surface that is in parallel to the optical axis in such parts constituting the aperture diaphragm is disclosed in, for example, Japanese Patent Application Laid Open Publication No. 2000-347239, Japanese Patent Application Laid Open Publication No. 11-52449, Japanese Patent Application Laid Open Publication No. 11-167140, and Japanese Patent Application Laid Open Publication No. 5-281591.
However, the conventional technology still has the following problems. FIG. 10 is a partially enlarged cross section of the aperture diaphragm illustrated in FIG. 8. As illustrated in FIG. 10, in a configuration in which the ND filters 53a and 53b are surface-bonded on the diaphragm blades 51a and 51b in parallel to each other, since end surfaces 55a and 55b of the ND filters 53a and 53b are in parallel to the optical axis A and the end surfaces 55a and 55b exist on the transmission section of the incident light when the lens aperture is small, the image quality is degraded because of the occurrence of the flare.
Generally used commercially available ND filters 53a and 53b are as thin as about 0.1 millimeter, and it is too thin to form the light shielding line on the edge surface. Therefore, the ND filters 53a and 53b are conventionally used in a state in which the end surfaces 55a and 55b are in parallel to the optical axis A, and the problem of the occurrence of flare cannot be solved.
According to a technique described in Japanese Patent Application Laid Open Publication No. 2000-347239, a moving distance of the diaphragm blade becomes long compared with a case in which the diaphragm blade is disposed vertically, and a size of the aperture diaphragm is increased. According to a technique described in Japanese Patent Application Laid Open Publication No. 11-52449, a secondary process, i.e., application of a chemical is required and the manufacturing process becomes complicated. According to a technique described in Japanese Patent Application Laid Open Publication No. 11-167140, since there is a danger that the ND filter comes off from a stopper, it is necessary to assemble the device while preventing the filter from coming off, and it takes time to assemble the device. According to a technique described in Japanese Patent Application Laid Open Publication No. 5-281591, since the ND filter is inclined and thus the diaphragm and the lens must be separated correspondingly, a size of the device in the optical axis direction is increased, which is not desirable in terms of the optical design.